The present invention relates to magnetic particles for a magnetic toner and a process for producing the same. More particularly, the present invention relates to magnetic iron oxide containing Fe.sup.2+ particles (magnetic Fe.sup.2+ -containing iron oxide particles) for a magnetic toner, which have an excellent fluidity and a high coercive force, which can suppress background development and, hence, produce a high resolution when the magnetic Fe.sup.2+ -containing iron oxide particles are used for a magnetic toner, and which have a high black chromaticity due to a high Fe.sup.2+ content. The present invention also relates to a process for producing such magnetic iron oxide particles.
A development process using, as a developer, composite particles which are produced by mixing and dispersing magnetic particles such as magnetite particles with a resin without using a carrier, in other words, what is called a one component magnetic toner is well known and generally used as one of the electrostatic latent image development processes.
With the recent improvement of the performances of copying machines such as a miniaturization of an electrostatic copying machine and an increase in the copying speed, the improvement of the properties of a magnetic toner as a developer has been keenly demanded. That is, a magnetic toner composed of small-diameter particles which can suppress background development and hence, produce a high resolution is in strong demand. Spherical magnetic particles which have conventionally been used have a low coercive force, so that when the magnetic particles are used for a magnetic toner composed of small-diameter particles, they are suffering from the following problem. Since the magnetic attraction is lowered, the toner is difficult to stir on a sleeve and difficult to be uniformly charged. As a result, the toner which is insufficiently charged causes background development.
To solve this problem, magnetic particles having a high coercive force and an excellent fluidity are now eagerly demanded.
Since the fluidity of a magnetic toner is largely dependent upon the surface state of the magnetic particles which are exposed to the surface of the toner, it is necessary that the magnetic particles themselves have an excellent fluidity. Angular magnetic particles such as octahedral and hexahedral magnetic particles have a poor fluidity, and when the angular magnetic particles are produced into a magnetic toner, the toner also has a poor fluidity. On the other hand, roundish magnetic particles have a good fluidity, and when the roundish magnetic particles are produced into a magnetic toner, the toner also has a good fluidity.
Therefore, roundish magnetic particles such as spherical magnetic particles, which can produce a magnetic toner having a good fluidity, are now required as a material.
It is known that the black chromaticity of magnetic particles is chiefly influenced by the Fe.sup.2+ content when the magnetic particles are magnetite particles having a size of about 0.1 to 0.5 .mu.m which are used for a magnetic toner, as described in pp. 239 to 240 of Powder and Powder Metallurgy, Vol 26, No. 7, as "The black chromaticity of a sample is influenced by the Fe(II) content and the average particle size, and powder having an average particle size of 0.2 .mu.m is bluish black powder, and it is the most suitable as a black pigment . . . . Every sample containing not less than 10% of Fe(II) has a black color although there is a slight difference in black chromaticity. If the Fe(II) content is lowered to less than 10%, the color of each sample changes from black to reddish brown."
Fe.sup.2+ -containing iron oxide particles having a high Fe.sup.2+ content and a high black chromaticity are, therefore, required.
Examples of the magnetic particles used as magnetic particles for a magnetic toner are octahedral magnetite particles (Japanese Patent Publication (KOKOKU) No. 44-668(1969)) and spherical magnetite particles (Japanese Patent Publication (KOKOKU) No. 62-51208(1987)). The conventional spherical and octahedral magnetite particles, however, do not have sufficient properties, as described in Japanese Patent Application Laid-Open (KOKAI) No. 3-201509(1991), as "The Fe.sup.2+ content of octahedral magnetite particles is about 0.3 to 0.45 in a molar ratio with respect to Fe.sup.3+, and although they are excellent in the black chromaticity, they have such a large residual magnetization that they are apt to cause magnetic cohesion, so that they have a poor dispersibility and they do not mix well with a resin . . . . Spherical magnetite particles have such a small residual magnetization that they are reluctant to magnetic cohesion, so that they have an excellent dispersibility and they mix well with a resin. However, since the Fe.sup.2+ content is about 0.28 at most in molar ratio with respect to Fe.sup.3+, the particles have a slightly brownish black color, in other words, they are inferior in black chromaticity . . . ."
Although hexahedral magnetite particles are proposed (Japanese Patent Application Laid-Open (KOKAI) No. 3-201509(1991)), since the hexahedral magnetite particles are angular, the fluidity cannot be said to be sufficient.
A manufacturing process including the step of adding silicon component during the reaction for producing magnetite in order to improve the properties of magnetite particles have conventionally been investigated. The processes proposed are, for example, a process (Japanese Patent Application Laid-Open (KOKAI) No. 5-213620(1993)) for producing magnetite particles comprising the steps of adding a silicon component to a solution of a ferrous salt, mixing 1.0 to 1.1 equivalents of an alkali with respect to iron to the resultant solution, carrying out an oxidation reaction while maintaining the pH at 7 to 10, adding iron in the middle of the reaction so that the iron is 0.9 to 1.2 equivalents based on the initial alkali, and carrying out an oxidation reaction while maintaining the pH at 6 to 10; and a process (Japanese Patent Publication (KOKOKU) No. 3-9045(1991)) for producing spherical magnetite particles by blowing an oxygen-containing gas into an aqueous reaction solution of a ferrous salt containing a ferrous hydroxide colloid which is obtained by reacting 0.80 to 0.99 equivalent of an alkali hydroxide with respect to Fe.sup.2+ by a two-staged reaction comprising the steps of adding 0.1 to 5.0 atm % of a water-soluble silicate (calculated as Si) based on Fe so as to produce magnetite nuclear particles and adding not less than 1.00 equivalent of an alkali hydroxide with respect to the remaining Fe.sup.2+.
The magnetite particles obtained by the above-described processes are, for example, magnetite particles (Japanese Patent Application Laid-Open (KOKAI) No. 5-213620(1993)) which contain a silicon component inside the particles, which have 0.1 to 2.0 wt % of a silicon component (calculated as silicon) based on the magnetite particles, exposed to the surface, which have the following BET specific surface area (m.sup.2 /g): EQU BET (m.sup.2 /g)=6/(particle size (.mu.m).times.5.2)+B,
and which satisfy the relationship B/A.gtoreq.30, wherein A represents the silicon abundance (wt %) exposed to the surfaces of the magnetite particles (calculated as silicon) based on the magnetite particles; and spherical magnetite particles (Japanese Patent Publication (KOKOKU) No. 3-9045(1991)) which have a bulk density of 0.40 to 1.00 g/cm.sup.3, which contain 0.1 to 5.0 atm % of Si based on Fe and which have an excellent temperature stability.
Various attempts have also been conducted for improving hexahedral magnetite particles. The improved magnetite particles are, for example, magnetite particles (Japanese Patent Application Laid-Open (KOKAI) No. 6-144840(1994)) which contain 0.10 to 5.0 wt % of a magnesium oxide (calculated as Mg) based on Fe, which have an average particle size of 0.10 to 0.25 .mu.m, and which have a substantially hexahedral shape with each face-like ridge, and which are obtained by adding a magnesium compound to an aqueous solution of a ferrous salt, and producing magnetite by blowing an oxidizing gas in the presence of a caustic alkali and/or an alkali carbonate under a weak acidic condition; and cubical magnetite particles (Japanese Patent Publication No. 6-53574(1994)) with chamfered edges which are obtained by reacting an aqueous solution of FeOOH, a ferric salt and an alkali at a raised temperature and precipitating the iron oxide from the solution in a temperature range of 80.degree. C. to the boiling point of the solution by using Na.sub.2 CO.sub.3.
However, the magnetite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 5-213620(1993) are produced by adding 1.0 to 1.1 equivalents of an alkali with respect to ferrous iron in a primary reaction, so that the magnetite particles obtained have a large particle distribution and it is impossible to obtain magnetite particles having a uniform particle size.
In the process of producing the magnetite particles described in Japanese Patent Publication No. 3-9045(1991), since the pH is not adjusted in a first-stage reaction and the pH is as low as less than 8.0, a large amount of sulfur is taken in during the reaction, so that the crystallizability is poor and the magnetic anisotropy in crystallization is low, which leads to a low coercive force of the magnetite particles produced.
The magnetite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 6-53574(1994) have a cubical shape with chamfered edges, but since some of them have a large particle size such as represented by the BET specific surface area as large as 0.5 to 5 m.sup.2 /g and they have an angular shape, they are inferior in fluidity.
The magnetite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 6-144840(1994) have a substantially hexahedral shape and each ridge of the hexahedron has a face, but since they are angular, they have a poor fluidity.
As described above, magnetic Fe.sup.2+ -containing iron oxide particles for a magnetic toner are now in the strongest demand, which are fine particles having a particle size of 0.05 to 0.30 .mu.m, which have a high coercive force so that the magnetic iron oxide particles display an excellent fluidity, suppress background development and, hence, produce a high resolution when the magnetic particles are used as magnetic toner particles having a small particle size, and which have an excellent black chromaticity due to a high Fe.sup.2+ content. However, no magnetic Fe.sup.2+ -containing iron oxide particles which have ever been produced, do not satisfy all of these conditions.
As a result of studies undertaken by the present inventors for solving the above-described problems, it has been found that by carrying out a process comprising carrying out a first-stage oxidation reaction for producing magnetic particles comprising blowing an oxygen-containing gas under heating, into an aqueous reaction solution of a ferrous salt containing a ferrous hydroxide colloid obtained by reacting an aqueous solution of a ferrous salt and 0.80 to 0.99 equivalent of an aqueous alkali hydroxide based on the ferrous salt, wherein not less than 0.9 atm % and less than 1.7 atm % of a water-soluble silicate (calculated as Si) based on Fe is added in advance to either of the aqueous alkali hydroxide and the aqueous solution of a ferrous salt containing the ferrous hydroxide colloid, and the pH of the aqueous reaction solution into which the oxygen-containing gas is blown in the first-stage oxidation reaction is adjusted to 8.0 to 9.5 at the beginning of the step of blowing the oxygen-containing gas; and carrying out a second-stage oxidation reaction for producing magnetic particles comprising after adding not less than 1.00 equivalent of an aqueous alkali hydroxide based on the residual Fe.sup.2+ to the aqueous reaction solution, blowing an oxygen-containing gas into the resultant aqueous reaction solution under heating, the obtained magnetic iron oxide particles for a magnetic toner have an average particle size of 0.05 to 0.30 .mu.m, have an excellent fluidity and a high coercive force, can suppress background development and, hence, produce a high resolution when the magnetic iron oxide particles are used for a magnetic toner, and have a high black chromaticity due to a high Fe.sup.2+ content. The present invention has been achieved on the basis of this finding.